May 17 (Reuters) - Amgen Inc won U.S. Food and Drug Administration approval on Thursday for the first drug in a new class designed to prevent migraine headaches in adults.

The drug, Aimovig, which is given monthly by self-injection, will have a list price of $6,900 a year, or $575 a month, the company said. Any discounts or rebates will depend on negotiations with health plans, Amgen spokeswoman Kristen Davis said.

Some Wall Street analysts had expected a price as high as $10,000 a year.

Amgen’s pricing strategy “appears to play into (an) appropriate framework of lower gross pricing,” Jefferies analyst Michael Yee said in a research note.

Express Scripts, the largest U.S. manager of prescription benefits, has called for Amgen to reconsider its strategy of setting a high list price for new drugs and then lowering the cost for health plans through hefty rebates.

Yee said Aimovig’s net price could fall to around $5,000 to $6,000 a year after rebates and discounts.

U.S. President Donald Trump last week blasted drugmakers and healthcare “middlemen” for making prescription medicines unaffordable for Americans as the administration said it would act to boost competition between pharmaceutical companies and test ways to pay for drugs based on their effectiveness.

The administration said longer-term priorities include restricting use of rebates, creating incentives for drugmakers to lower list prices, and investigating tools to address foreign government practices that it said could be harming innovation and driving up U.S. prices.

Aimovig is the first in a new class of treatments designed to prevent migraine by interfering with calcitonin gene-related peptide (CGRP), which is involved in the processes that kick off the severe headaches, such as dilation of blood vessels in the brain.

Preventive medications may be an option for around eight million Americans suffering from migraines, Amgen said.

In three separate pivotal trials, involving patients with chronic or episodic migraines, patients given Aimovig experienced an average of one to 2.5 fewer migraine days a month than those treated with a placebo, the FDA said.

The most common side effects reported by patients in the trials were injection site reactions and constipation.

Current migraines treatments are mostly repurposed generic drugs including anti-depressants and hypertension medicines. Allergan’s anti-wrinkle injection Botox is approved for patients suffering from 15 or more migraine headaches a month.

Analysts, on average, have forecast annual Aimovig sales of nearly $1 billion by 2022, according to Thomson Reuters I/B/E/S.

Express Scripts has also suggested that Aimovig prescriptions be at least partially refunded if the drug fails to work, since many patients in the clinical trials did not have a significant response to the treatment.

Amgen, which will market Aimovig in partnership with Novartis, said the drug’s price “reflects the value it brings to patients and society.” The company said it will pay most out-of-pocket costs for eligible patients with commercial insurance. (Reporting by Deena Beasley; Editing by Leslie Adler and Grant McCool)